Internal shoe drum brakes include at least one pair of brake shoes which are oppositely arranged with respect to one another within a brake drum in such a manner that one pair of adjacent ends of brake shoes is pivotally mounted on a back plate while the other pair of adjacent ends engage with an expander. Upon rotation of the expander the ends of the brake shoes engaged therewith will be mutually displaced and brake linings on the outer shoe surfaces caused to contact the inside surface of the brake drum. Such type of drum brakes is described e.g. in British Patent Specification No. 1,194,743 which is herewith incorporated by reference. Contact between brake shoe linings and brake drum surface results in a drag of retarding torque which is the greater, the greater the force transmitted by the expander.
A constant relation between angular displacements and retarding torques is a basic requirement with respect to brakes in general and to vehicle brakes in particular since then same reactions on a driver's part have the same braking effects and, thereby, less mental concentration is needed during driving which is a considerable factor with heavy duty vehicles such as trucks and busses. However, the required constancy is, as a rule, frustrated by the behaviour of brake linings and their counterparts. Readjustments to eliminate a wearing away of contacting surfaces has been proved insufficient for reestablishing an originally adjusted relation between braking force and retarding torque because, in addition to their sizes, also the physical properties of contacting pairs undergo a change which is due to frequent mechanical and thermal loads caused by braking operations.